Slope currents and contourites in an eastern boundary current regime: California Continental Borderland

Analysis of piston cores from lower slopes of central and outer basins of the California Continental Borderland shows the presence of structures ranging from starved silt ripples and lenses to erosion surfaces and truncated burrows to cross-bedded units that testify to reworking of the sediments by bottom currents. Adjacent basin-floor piston cores do not reveal these structures, but exhibit the usual bioturbated hemipelagic mud and turbidites. Slope sediments generally contain more silt than the adjacent basin-floor clay silts. The slope grain-size distributions are multimodal as a result of mixing of hemipelagic mud and thin (millimetre scale) silt layers as a consequence of the sampling intervals at centimetre scale. Carbonate and organic carbon contents tend to be low during the glacial periods but variations from this pattern occur that are probably related to the shift in the upwelling associated with the California Counter Current between interglacial and glacial conditions. The evidence for reworking is most abundant from late in Marine Isotope Stage 5 (MIS 5) to early Stage 4 (MIS 4), and it decreases into the Holocene. No structures are observed in the Holocene sections of the slope cores. The temporal distribution of the reworking structures can be explained either by changes in the degree of bioturbation (bottom-water oxygenation), which would work to erase the structures, or variation in the intensity of lower slope circulation through time. The observed pattern is in rough agreement with documented changes in bottom-water oxygenation conditions for these basins. However, the occurrence of reworking is also dominantly within glacial intervals, where lower sea levels produce stronger circulation within the basins as a result of exposure of banks and resulting restriction of cross-sections of the deeper flow pathways. These observations add to the increasing evidence that sediment transport by bottom currents is not restricted to the intensively studied western boundary current drift deposits. The action of bottom currents on eastern boundary slopes can introduce subtle effects on what are commonly assumed to be continuous records.